In general, a variety of compressors have been developed in order to be used for the compression of refrigerants in vehicle cooling systems. These compressors are classified into reciprocating compressors which perform compression during reciprocating and rotary compressors which perform compression during rotating, according to methods of compressing refrigerants.
When the temperature of outdoor air is low as in the change of seasons or the winter season, a refrigerant has a temperature equal to or less than the boiling point, and is thus changed from gas phase to liquid phase. A compressor may functionally compress a gas-phase refrigerant but may not compress a liquid-phase refrigerant. For this reason, when a liquid-phase refrigerant is introduced into a compression system, fatigue may be gradually accumulated in the compression system due to the resistance of fluid.
When the compressor of an air conditioning system begins to compress a liquid, a refrigerant is changed from liquid phase to gas phase while the pressure and enthalpy of the refrigerant are increased, and a time required for this phase change has a large effect on the durability of the compressor. In addition, the liquid refrigerant adversely affects lubrication since it basically washes oil in the compressor. Furthermore, since oil separation is not properly performed if the refrigerant is insufficiently evaporated at the discharge end of the compressor, an oil separation system may not be normally operated.
For the durability of the compressor of the conventional air conditioning system, it is tested whether or not the drive unit of the compressor satisfies durability standards by testing the operation of liquid compression for several thousands of cycles. Accordingly, since the drive unit must be necessarily designed to have stiffness in order to satisfy this durability, the method of preventing the operation of liquid compression in the compressor is urgently required.